This invention relates to computed tomography (CT) systems and specifically to an anti-aliasing filter for use in processing the data collected by a CT system.
In a computed tomography system, an x-ray source is collimated to form a fan beam with a defined fan beam angle. The fan beam is orientated to lie within the x-y plane of a Cartesian coordinate system, termed the "imaging plane", and to be transmitted through an imaged object to an x-ray detector array oriented within the imaging plane.
The detector array is comprised of detector elements separated by a pitch approximately equal to their width. Each detector element measures the intensity of transmitted radiation along a beam projected from the x-ray source to that particular detector element. The intensity of the transmitted radiation is dependent on the attenuation of the x-ray beam along that ray by the imaged object.
The x-ray source and detector array may be rotated on a gantry within the imaging plane and around the imaged object so that the angle at which the fan beam intersects the imaged object constantly changes. As the gantry rotates, a number of projections forming a projection set are acquired, each projection made up of the intensity signals from the detector elements as they travel over a small angle of gantry rotation centered around a projection angle.
The acquired tomographic projection sets are typically stored in numerical form for computer processing to "reconstruct" a slice image according reconstruction algorithms known in the art. A projection set of fan beam projections may be reconstructed directly into an image by means of fan beam reconstruction techniques, or the intensity data of the projections may be sorted into parallel beams and reconstructed according to parallel beam reconstruction techniques. The reconstructed tomographic images may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
The continuous rotation of the gantry produces a constantly changing signal from each detector corresponding to the variation of attenuation of the x-ray beam associated with that detectors as the angle of the x-ray beam changes. This signal may be integrated over the increment of gantry rotation associated with each projection angle to produce the detector signal for that projection angle. This integrated value is then held for sampling and conversion to a digitized detector value by a data acquisition system ("DAS") for storage and reconstruction by a computer.
The integration of the detector signal increases the sensitivity of the detectors and also provides an intrinsic bandlimiting of the detector signal to prevent "aliasing" during the sampling of the detector signal by the DAS. As is understood in the art, aliasing is a signal artifact produced by frequency components in a sampled signal having a frequency higher that half the sampling rate.
In order to provide adequate time for the sampling of each detector signal by the DAS, two such integrators may be used with each detector element. One integrator holds the value of previously integrated data for sampling while the other integrator integrates new current data from the detector element. This two integrator design is termed "integrate and dump" and has the advantage of providing a well defined integration time and a generous sampling window for the DAS.
Nevertheless, the integrate and dump circuit is susceptible to variations in gain caused by changes in the value of its integrating capacitor. Further, the solid state switches typically used to alternately connect the two integrators have significant leakage currents and require that the detector signal first be preamplified. Variations in the gain of this independent preamplifier contributes to the gain variations experienced with the integrate and dump design. Variations in gain can cause unacceptable streaking, "o-rings", smudges or other artifacts in the reconstructed tomographic image.
A continuous wave filter, such as a low pass filter, may be used in place of an integrator. In a continuous wave filter the filter output reflects the previous detector signal on a weighted rolling basis. The use of a low pass filter with an appropriate frequency cutoff point eliminates aliasing.
The operation of the continuous wave filter is such that sampling may occur at any time, provided an appropriate correction is made for gantry position, and hence only a single filter is required for each detector. This eliminates the need for an independent preamplifier associated with the solid state switches of the integrate and dump circuit.
The use of a continuous wave filter provides improved gain stability over the integrate and dump design. Direct current feedback may be established around the continuous wave filter (unlike an integrator) and therefore, the gain of the filter may be fixed by a single resistor as opposed the capacitor of the integrate and dump circuit and the resistor of its associated preamplifier. Resistors are generally more stable than capacitors and one element is generally more stable than multiple elements.
With certain CT imaging techniques, the x-ray beam switches rapidly between two beam intensities or two beam positions during the rotation of the gantry. In dual energy scanning, for example, the power to the x-ray tube may be varied to a produce two x-ray beams having different spectra to create two images whose comparison may be useful for distinguishing between various tissue types. Alternatively, in "spot wobble" scanning, the point of x-ray emission may be "wobbled" with respect to the gantry to create two beams with slightly different angles to increase the resolution of the x-ray image. This latter technique is described in detail in U.S. application Ser. No. 07/540,995 filed Jun. 20, 1990, entitled: "Computed Tomography System with Translatable Focal Spot", assigned to the same assignee as the present invention and hereby incorporated by reference.
In each of these dual beam techniques, the beam is rapidly shifted between states as the gantry rotates so as to lessen the effects of movement by the patient on the consistency of the data collected. Such patient movement generally causes more variations between corresponding detector signals when the state of the beam is shifted only between full revolutions of the gantry.
With the rapid shifting of the x-ray beam, the signal from the detectors also changes and must be separated in synchronism with the shifting of the beam so as to collect two distinct sets of data, one associated with each beam state. One way to separate the detector signals associated with each state of the beam is to use the previously described dual integrators of the integrate and dump circuit. The first integrator is adjusted to integrate (and hence to collect data) only during the first state of the beam, and the second integrator is adjusted to integrate only during the second state of the beam. The outputs of the integrators produce two distinct sets of detector data one associated with each beam state.
Unfortunately, this approach still carries the drawbacks of gain sensitivity associated with the integrate and dump circuit as previously described, i.e. the gain of the circuit is determined by a relatively unstable capacitor value associated with each integrator and a resistor associated with a separate preamplifier.
The continuous wave filter, previously described, is not suitable for these dual beam techniques because the filter produces a continuous output that is a function of previous detector signals regardless of the beam state. The continuous nature of the continuous wave filter, which previously worked to its advantage by allowing flexible sampling, prevents clean separation of the two beam signals.